1. Field of the Invention
The present invention relates to a rechargeable zinc electrode used as a negative electrode in a storage cell having an alkaline electrolyte, and specifically to Nickel-Zinc and Silver-Zinc cells.
2. Background Art
Rechargeable cells such as, for example, Nickel/Zinc, Silver/Zinc, Zinc/Air and Manganese-dioxide/Zinc, containing zinc electrodes are of significant interest due to the abundance and, therefore, low cost of zinc, as well as low equivalent weight, high coulombic efficiency, reversible electrochemical behavior, and reduced environmental disposal problems (e.g., in comparison to lead or cadmium).
The electrode potential of the zinc electrode in alkaline solutions is such that the electrode corrodes due to hydrogen evolution and conversion of zinc to zinc oxide. This leads to high self discharge of the cell, cell leakage due to pressure build-up inside the cell and poor performance of the cells. To reduce the corrosion and hydrogen evolution, several materials (for example HgO, PbO, SnO2, Tl2O3, CdO, In(OH)3, Ga2O3 and combinations thereof) have been added to the zinc electrode.
Until the 1970's the additive of choice had been HgO, which is excellent at suppressing hydrogen evolution. However, HgO is toxic, costly to dispose of and has come under increasing regulatory pressure. These issues created an increased concern which inspired the researchers and manufacturers to find a suitable substitute for HgO. As a result, all toxic materials (e.g., HgO, PbO, CdO etc.) have been completely removed from gelled Zinc electrodes used in primary MnO2/Zn alkaline cells. Unfortunately, because of the requirements of rechargeability, these compositions can not be used directly in rechargeable electrodes. Consequently, the rechargeable zinc electrode, still contain some toxic ingredients, as exemplified below.
Many inventors have reported different compositions for the rechargeable Zinc electrodes. The composition typically has Zn-oxide as the major component, a metal oxide (such as Lead oxide, cadmium oxide, bismuth oxide) as hydrogen suppressant, and a binder (polystyrene, methyl cellulose, polytetrafluoroethylene (Teflon), polyvinylalcohol, Cellulose, etc.). Himy et al. [A. Himy & O. C. Wagner, U.S. Pat. No. 4,327,157 dt. Apr. 27, 1982] describe a rechargeable Zinc electrode containing a corrosion inhibitor such as PbO, SnO2, Tl2O3, In(OH)3, or mixtures thereof (All described compositions contain various amounts of PbO and another compound). Adler et al. [T. C. Adler, F. R. McLarnon and E. J. Cairns, Journal of the Electrochemical Society, Vol. 140, p. 289 (February 1993)] describe an electrode containing 94% Zinc-oxide, 2% Lead oxide, 1% newsprint and 4% Teflon as binder. Jones [U.S. Pat. No. 4,358,517, dated Nov. 9, 1982] describes a zinc electrode containing 0.25 to about 1.5 moles of Calcium hydroxide per mole of Zinc oxide (active ingredient), lead oxide (hydrogen suppressant) content up to about 4% by weight of the mixture, and cellulose (as binder) of about 0.5% to about 10% by weight of the mixture. Charkey [U.S. Pat. No. 5,460,899 dated Oct. 24, 1995] describes a zinc electrode with 5-20% of metal (Pb, Bi, Cd, Ga or Tl) oxide, 15-40% Calcium hydroxide, 5% Teflon as binder and rest Zinc oxide as active component. Serenyi [U.S. Pat. No. 5,773,176 dated Jun. 30, 1998] describes an electrode containing bismuth-oxide and other additives e.g., lead oxide and cadmium oxide with zinc oxide. Coonan et. al. [E. W. Coonan, P. D. Frayer, R. J. Monahan & K. A. Wexell, U.S. Pat. No. 6,284,192 dated Sep. 4, 2001] describe a zinc electrode containing 8% lead oxide. Thus, most of the rechargeable zinc formulations still contain lead and/or cadmium oxide, both of which are toxic materials. Therefore, there is a need for a formulation for rechargeable zinc electrode which is free of toxic materials. It is essential that the electrode performance be at least as good as the performance of prior arts electrodes containing lead and or cadmium
The main objective of the invention is to make a zinc electrode free of any toxic materials. Another objective of the invention is to obtain a performance better than any other prior art zinc electrode.
These and other objectives of the invention will become apparent in light of the present specification, claims, and drawings.